In optical telecommunications networks, optical amplifiers are widely used to amplify optical data signals. In an optical amplifier having an inverted erbium doped fibre (EDFA) an input optical data signal is amplified by stimulated emission. Typically, the users seeks to amplify the optical data signal either by a set gain, which is known as gain mode amplification, or to a set power level, which is known as power mode amplification. The level of amplification is commonly, although not exhaustively, determined by sampling the optical output from the optical amplifier, and using the data in a feedback arrangement to generate a control signal to control the output power of a pump laser that pumps the optical inversion of the EDFA.
As well as amplification of the optical data signal, the EDFA generates a noise component through amplified spontaneous emission (ASE). The optical data signal is typically centred on one or more wavelengths corresponding to the channels standardised by the International Telecommunications Union (ITU). In contrast, the ASE is typically generated across a much broader wavelength range, e.g. around 40 nm, which is a substantial part of the gain bandwidth region of the amplifier. The level of ASE depends upon the optical data signal gain within the Er fibre, inversion and temperature of the EDF. Further, the level of ASE produced by an optical amplifier also varies due to the loss variability of optical components since passive losses in the amplifier affect the gain required in the erbium fibre.
Disadvantageously the level of the ASE in the optical output from an optical amplifier cannot be easily optically determined, since the optical power detectors (e.g. photodetectors) typically used in optical amplifiers are relatively wavelength insensitive (and necessarily so, in the case in which the optical amplifier should be adapted for operation at a range of ITU channels), and so detect both the amplified optical data signal and the ASE. Accordingly, disadvantageously, the presence of the variable amount of ASE leads to incorrect amplification of the optical data signal, which consequently increases detection errors in an associated optical network.
The following approaches are known which seek to address this problem.
In a fixed, single channel optical amplifier, it is known to use a fixed wavelength discriminating filter to filter out the ASE that arises at wavelengths away from the bandwidth of the optical data signal. Disadvantageously, such an approach is inflexible such that the optical amplifier can only be used for a fixed, defined wavelength of the filter and a different optical amplifier would need to be manufactured for each signal channel.
A further disadvantage of using a fixed wavelength discriminating filter is that this approach cannot be applied to optical amplifiers in systems that handle light at more than one channel, e.g. in optical networks comprising transmitters that use tunable laser optical sources. Although tunable filters are known, such components are too substantial in size and too costly to be commercially viable for deployment in many applications of commercially manufactured optical amplifiers at the current time.
A second approach requires each optical amplifier to be fully characterised for ASE over a large range of operating conditions prior to sale, so that the optical amplifier can be sold with a large, unique table of data, giving the levels of compensation that are required to ensure correct amplification of the optical data signal. This is particularly so for a single channel amplifier where there can be many unique operating input conditions of wavelength and optical signal data input power as well as a large number of unique gain or output power requirements. The ASE compensation data may be provided in a look-up table stored in the optical amplifier's operating system, or provided separately. Disadvantageously, full characterisation of every optical amplifier from a production line is time consuming, requires the deployment of a substantial resource of expensive characterisation equipment, and consequently substantially increases the manufacturing cost of the optical amplifiers.
A third approach seeks to estimate the level of ASE produced by use of an empirically derived equation, and applies a corresponding compensation coefficient to the gain of the optical amplifier, to adjust the output power of the optical data signal to the desired level. Such an approach is disclosed in U.S. Pat. No. 6,519,081. This technique works well when there is a known and consistent set of input optical signals, but, disadvantageously, the level of ASE produced does not conform to a simple empirical equation with respect to the variation of ASE level as a function of input optical signal wavelength.
Accordingly, there is a need for an improved method of operating an optical amplifier, which provides more accurate amplification of an optical data signal by a desired gain level or to a desired optical power level.